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Building a good long-term portfolio isn't simple. Its foundation depends on a number of things, including sound decision-making. But sometimes it's hard to hard to figure out which decisions need to be prioritized. So we asked four Motley Fool contributors to tell us which decisions investors should be making in 2015. Read on to learn what they think.

Selena Maranjian
One of the most important investing decisions you should make in 2015 -- and every year, really -- is whether you're invested in your best ideas. In other words, go through all the holdings in your portfolio and ask yourself whether each holding has changed for the better or the worse and whether it's still among your best investments. If you've begun to lose faith in a stock's fundamentals, then it may be a good time to consider selling.

Think of it this way. Let's say that based on all that you've read and learned about hundreds or thousands of stocks, you have 15 to 20 whose futures you're most excited about, whose growth prospects you're most confident in, and whose financial health you're most convinced of. Given that those are your best ideas, they should be the ones in which you've invested all or most of your long-term dollars. Why put any money in your 39th-best idea? It's smarter to just add more money to your top ideas.

If you don't have a list of your favorite stocks, it's worth developing one. This exercise will help you see your portfolio more clearly and can lead you to concentrate on investments that you think are most likely to grow. Diversification is important in a portfolio, but you needn't overdiversify into dozens or hundreds of stocks. As Warren Buffett, arguably the world's best investor, has said, "Wide diversification is only required when investors do not understand what they are doing."

Matt Frankel
Given that we've been in a bull market for about six years now without any significant corrections, I can't help worrying that 2015 will be the year when we finally see the market correct itself. So my most important investment decision for 2015 is how to prepare for a market dip while still positioning myself to profit if stocks continue to climb upward.

The best way to do this is to shift your focus away from momentum stocks and toward stable, high-quality, low-beta stocks, which tend to outperform the market in bad times and over the long run.

A great place to start is with the Dividend Aristocrats, a group of stocks that have increased their dividends for at least 25 consecutive years. Stocks such as Johnson & Johnson (NYSE:JNJ) and Procter & Gamble (NYSE:PG) have a long track history of outperforming the market, particularly in turbulent times. Just look at how they did relative to the S&P during the 2008-2009 crash:

JNJ Chart

JNJ data by YCharts.

Another excellent stock to consider is Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B), because it's like buying dozens of rock-solid defensive stocks in one. Berkshire won't make you rich overnight, but it's a great stock to own when things get rough, and it has delivered unbeatable performance over the long run.

Dan Caplinger
One key decision investors need to make involves looking at their mix of taxable and tax-favored investment accounts to see whether they're well positioned for long-term tax trends. Until now, the trend has been toward higher tax rates. The restoration of the 39.6% tax bracket and new surtaxes on earned income and investment income have sent marginal rates on the highest-earning taxpayers to their highest levels in decades. Those high rates have made contributions to retirement accounts more lucrative for high-bracket taxpayers.

With Republicans taking full control of Congress, however, some believe that the trend toward higher taxes could be coming to an end. Despite aggressive proposals from the Obama administration, taxes aren't likely to rise further, and as a result, many investors should be considering strategies that could benefit from lower-tax environments in future years. For instance, if you're thinking about a Roth conversion, it might be prudent to wait a while to see whether lower tax rates will make switching to a Roth a bit cheaper at tax time. Moreover, by taking advantage of lower rates on dividend income and long-term capital gains in taxable brokerage accounts, you can reduce your total tax burden and leave yourself more room to use your tax-sheltered investment capital for the stocks and other investments with the greatest capacity for capital appreciation.

Jason Hall
The most important decision investors must make this year is the same one they face every year: What should I avoid investing in?

There is a lot of evidence that most investors' underperformance is a product of our own mistakes -- or, as Morgan Housel has described them, "unforced errors." Unless you've demonstrated a track record of being able to make great picks and outperform the rest of the market on a regular basis, you're better off simply trying not to mess up.

As a starting point, this means that most of our focus as investors needs to be on what we can control, like maximizing contributions to tax-advantaged 401(k)s and IRAs first and taxable accounts second.

It also means you should probably forgo the risky biotech start-up that the self-proclaimed office expert tells you is a "can't-miss double in the next month," and instead invest for the long term in companies with a proven record of strong returns and solid long-term potential.

I admit that it's more exciting trying to catch the next big thing, but you'll probably sleep better if you watch the fireworks from a distance. Furthermore, a mountain of evidence suggests you'll make a lot more money than your day-trading coworker. Focus on what you can control and avoid those avoidable mistakes.